There is known a camera system that enables, by using a camera mounted on a rear side of a vehicle, to display an image behind the vehicle, where it is hard for a driver to view, on a display unit inside the vehicle. For such a system, there is known a method, by adjusting a difference between an assumed image capturing a subject whose position information is known and an actual image of the subject actually taken by the camera, to obtain parameters for calibration of a camera mounting position (for example, see Patent Document 1 and Patent Document 2).